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The oil in poisonous plants can stay active for a few years! That's why these dead plants are just as dangerous as the live ones!

What are poison ivy, oak, and sumac?
Poison ivy, poison oak, and poison sumac are plants that can cause a skin rash called allergic contact dermatitis upon contact. The red, uncomfortable, and itchy rash often shows in in lines or streaks and is marked by

fluid-filled bumps (blisters) or large raised areas (hives). It is the most common skin problem caused by contact with plants.

What causes the poison ivy rash?
Urushiol is the chemical in poison ivy plants that causes the typical allergic reaction and symptoms of poison ivy rashes. Although it can sometimes be washed off within 10 minutes, after that, it is very likely to cause a reaction within 8 to 48 hours. Urushiol is found in the leaves, stems, and roots of poison ivy plants, which means that you can get a rash even in the winter, when a plant has lost all of its leaves. You can get the oil on your skin by:
touching the poison ivy plant.
touching any clothing, including shoes, that have come in contact with the plant.
touching any gardening tools that may have the oil on it.
touching any outdoor pets that have been around the poison ivy and have gotten the oil on their hair.
burning the poison ivy plant.  The oil from the plant is carried in the smoke.

What are the symptoms of the rash?
The usual symptoms are:
Itchy skin where you came in contact with the plant.
Red streaks or lines where the plant brushed against the skin, or general redness.
Small bumps or larger raised areas (hives)
Blisters filled with fluid that sometimes leaks out.

The rash usually appears 8 to 48 hours after your contact with the urushiol. The rash will continue to develop in new areas over several days but only on the parts of our skin that had contact with the urushiol or those parts where the urushiol was spread by touching.

The rash is not contagious. You cannot catch or spread a rash once it appears, even if you touch it or the blister fluid, because the urushiol will already be absorbed or washed off the skin.  The rash may seem to be spreading, but either it is still developing from earlier contact or you have touched something that still has urushiol on it. The more urushiol you come in contact with, the more severe your skin reaction.

How is the rash treated?
Most poison ivy, oak, or sumac rashes can be treated successfully at home. Initial treatment consists of washing the area with water immediately after contact with the plants.  To relieve symptoms, use wet compresses and take cool baths.  Nonprescription antihistamines and calamine lotion also may help relieve symptoms.  Moderate or severe cases of the rash may require treatment by a health professional.

How can I prevent the rash from poison ivy, oak, and sumac?
The best way to prevent the rash is to learn to identify and avoid the plants.  When contact with the plants is unavoidable, heavy clothing (long pants, long-sleeved shirt, and vinyl gloves) may provide some protection.

Plant Characteristics:
Poison Ivy
It is an upright, climbing, or trailing shrub that bears small yellowish-white flower clusters. Its stems are covered with fibrous roots, making them look hairy. The leaves are divided into three glossy or dull green leaflets. Every part of the plant contains a volatile oil (urushiol) that can cause severe inflammation of the skin, itching, and blistering when touched. Most people are allergic to urushiol. It is a very hearty plant that spreads rampantly.

Poison Oak Western poison oak is similar to poison ivy in appearance. However leaflets of western poison oak have rounded tips, whereas leaflets of poison ivy have tips that are pointed.

Poison Sumac
Poison Sumac is a "tall shrub or small tree with 6-12 leaflets arranged in pairs, and an additional single leaflet at the end of each midrib. The small yellowish green flowers borne in clusters, mature into whitish green fruits that hang in loose clusters 10-30cm in length. The male and female flowers of poison sumac are on separate plants, as in poison ivy and western poison oak. Although nonpoisonous sumac species have leaves similar to those of poison sumac, the nonpoisonous species have red fruits that form distinctive, erect, cone-shaped terminal heads, not the hanging whitish green fruits of poison sumac."
 

Poison Ivy, Oak and Sumac (American Academy of Dermatology)

Poison Ivy rash and leave illustration (National Institute of Health)